Precision instruments such as a scanning tunneling microscope (STM) generally operate with ultra high vacuum. Positioning and repositioning of samples is carried out remotely and requires high accuracy and special precautions for the in-vacuum procedures. Similarly, repositioning of components of the instrument may require high precision of placement. The STM, for example, can analyze surfaces down to atomic accuracy and has some very small components including a metal probing tip mounted only 0.1 mm above the sample surface. Descriptions of some of the mechanical requirements for a STM are given in an article "Vibration Isolation for Scanning Tunneling Microscopy" by M. Okano, K. Kajimura, S. Wakiyama, F. Sakai, W. Mizutani, and M. Ono, J. Vac. Sci. Technol. A5(6), 3313-3320 (1987).
The probing tips are replaced frequently, preferably without interrupting the vacuum. Therefore, a carrier which is located in the chamber and can carry a number of tips is desirable. In replacing a tip the carrier must be moved to a position adjacent to the tip holder with a considerable degree of accuracy. The carrier may have positions for the tips arranged in a circle so that selection of a tip is effected by a selected orientation of the carrier. Thus the carrier must be positioned linearly into a seated position which also ensures an accurate orientation. Other instruments have similar requirements for positioning. Therefore, an object of the invention is to provide a novel mechanism for precise positioning of a carrier in an apparatus. Another object is to provide such a mechanism which positions linearly as well as rotationally. A further object is to provide such a mechanism which allows selection from among a plurality of rotational positions of the carrier.